1. Field of the Invention
The present invention relates to a novel chamber effluent monitoring system and a semiconductor processing system which include an absorption spectroscopy measurement system for measuring a gas phase molecular species. The present invention also relates to a method of detecting a gas phase molecular species within the inventive chamber effluent monitoring system and inventive semiconductor processing system.
2. Description of the Related Art
In the manufacture of semiconductor integrated circuits (ICs), it is important to have an extremely low partial pressure of molecular impurities in the processing chamber. In particular, water vapor is especially detrimental to the devices fabricated in the processing tools. For example, water vapor must be eliminated or minimized in an aluminum etching chamber in order to achieve reproducible etching processes. Also, when subjected to water vapor during processing, corrosion of the device metallization layers is accelerated, substantially reducing product yield.
Molecular impurities may be introduced into the processing chamber in a number of ways. For example, molecular impurities may be present in the process gases introduced into the chamber during processing. Also, molecular impurities such as moisture are present in the air to which the chamber is exposed during maintenance of the processing tool. Air and water may also be introduced into the processing chamber whenever a substrate is introduced into the chamber. Molecular impurities may also be released from the substrates themselves after introduction into the process chamber or may result from the process conditions themselves. For example, during plasma processing and rapid thermal processing, molecular impurities may take the form of reaction byproducts or, as in the case of water vapor, may be released from substrate and chamber surfaces upon heating.
Molecular impurities which are introduced into the process chamber with a substrate are typically removed by purging the chamber with a pure gas, by evacuating the chamber, or by a series of pressurization-evacuation cycles.
In the case of chamber evacuation, the base pressure in the chamber is used as a measure of the extent of removal of the molecular impurities. Conversely, when relying on the chamber purge technique, the chamber is filled with a pure gas for a period of time which is usually determined by the operator's experience.
The extent of removal of atmospheric contamination from the processing chamber can also be determined by the measurement of water vapor concentration in the chamber. Such a technique is particularly useful in the case of contamination resulting from exposing the processing chamber to the outside atmosphere during maintenance and from introducing a substrate into the chamber. Water vapor can adhere to the surfaces inside a processing chamber as well as to the surface of the substrate. It is present in the atmosphere in an amount of from about 1-2, and is generally the most difficult atmospheric constituent to remove by evacuation or purging.
In state-of-the-art production facilities, particle monitors are often used to monitor particulate contamination in situ. It is known to dispose particle monitors in the exhaust line of processing tools. (See, e.g., P. Borden, Monitoring Vacuum Process Equipment: In Situ Monitors--Design and Specification," Microcontamination, 9(1), pp. 43-47 (1991)). While such particle monitors may be useful for tracking process events which result in the generation of particles, they cannot be used to monitor molecular concentrations.
Among the analysis tools which can be used in the measurement of molecular contamination is one type of mass spectrometer, usually referred to as a residual gas analyzer (RGA). (See, e.g., D. Lichtman, Residual Gas Analysis: Past, Present and Future, J. Vac. Sci. Technol., A 8(3) (1990)). Mass spectrometers generally require pressures in the range of about 10.sup.-5 torr, whereas the operating pressures of semiconductor processing tools are often at pressures in the range of from about 0.1 to 760 torr. Consequently, mass spectrometers require sampling systems and dedicated vacuum pumps. Mass spectrometers are generally both expensive and not compact in construction. Moreover, the differentially pumped chamber in which the mass spectrometer is housed contributes a high level of residual water vapor which is difficult to remove and which severely limits the sensitivity of the mass spectrometer for water vapor measurement.
Optical emission spectroscopy is widely used for monitoring plasma processes. In principle, optical emission spectroscopy can be used to monitor molecular contamination in the processing tool. However, the optical emission spectrum is very complicated, and this method cannot be used in non-plasma processes.
Other spectroscopic techniques have been widely used in research situations to study process chemistry. (See, e.g., Dreyfus et al., Optical Diagnostics of Low Pressure Plasmas, Pure and Applied Chemistry, 57(9), pp. 1265-1276 (1985)). However, these techniques generally require specially modified process chambers and have not generally been applied to the study of contamination. For example, the possibility of in situ moisture monitoring by intracavity laser spectroscopy has been mentioned generally in a review of that technique. (See, e.g., G. W. Atkinson, High Sensitivity Detection of Water via Intracavity Laser Spectroscopy, Microcontamination, 94 Proceedings Canon Communications (1994)).
Finally, conventional gas analyzers have been applied to in situ moisture measurement, usually for processes running at or close to atmospheric pressure. (See, e.g., Smoak et al., Gas Control Improves Epi Yield, Semiconductor International, pp. 87-92 (June 1990)). According to such techniques, a portion of the process gas is extracted into a probe which then delivers the sample to the analyzer. However, use of a probe is undesirable in the measurement of moisture since moisture tends to adsorb on the surfaces of the probe. Moreover, this approach is often impractical as it requires considerable space to accommodate the conventional gas analyzers. It is well known that free space inside a semiconductor fabrication cleanroom is typically at a minimum.
A method for measuring the instantaneous moisture concentration and drydown characteristics of a processing environment is disclosed in U.S. Pat. No. 5,241,851, to Tapp et al. According to this method, a moisture analyzer alternately samples the effluent from a process chamber and the gas generated by a standard gas generator. The output of the standard gas generator is adjusted until the analyzer indicates no difference between the effluent and standard gas streams. Because the moisture content in the output of the standard gas generator is known, the level in the effluent stream can be determined. This system, however, is inconvenient and complicated as it requires a standard gas generator and complicated piping to effect switching between the effluent and standard gas streams. Moreover, there is a risk of backflow from the standard gas generator to the process chamber, resulting in contamination.
To meet the requirements of the semiconductor processing industry and to overcome the disadvantages of the prior art, it is an object of the present invention to provide a novel chamber effluent monitoring system, and in particular a novel semiconductor processing system, which includes an absorption spectroscopy system for detecting gas phase molecular impurities, which will allow for accurate, instantaneous and in situ determination of gas phase molecular impurities in a semiconductor processing tool.
It is a further object of the present invention to provide a method of detecting gas phase molecular species within the inventive chamber effluent monitoring and semiconductor processing systems.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.